Fire protection and suppressant systems may operate by detecting the presence of smoke and other airborne pollutants. Upon a threshold level of smoke being detected, an alarm may be activated and operation of a fire suppressant system may be initiated. While the fire itself will cause damage, considerable damage can also be caused by operation of the fire suppression system and subsequent removal of the suppressant can be quite hazardous. Many traditional suppressants, such as halon, are also ozone depleting whereby this use is environmentally undesirable. A detection system which is sufficiently sensitive to detect an abnormal condition prior to the onset of a fire is very advantageous as it enables action to be taken at a very early stage before the onset of actual fire conditions. For example, when most substances are heated, even before heating occurs to a point at which a fire commences, emissions will be generated and if these can be detected by a very sensitive system, a warning provided at that very early stage may allow the problem to be detected and rectified, or the equipment turned off, before the fire actually starts.
The detection system may incorporate a sampling pipe network consisting of one or more sampling pipes with sampling holes installed at positions where smoke or pre-fire emissions can be collected. Air is drawn in through the sampling holes and along the pipe by means of an aspirator or fan and is directed through a detector at a remote location.
Although there are a number of different types of smoke detectors which can be used as the detector in a system as outlined above, one particularly suitable form of detector for use in such a system is an optical scatter detector which is able to provide good sensitivity at reasonable cost. Optical scatter detectors operate on the principle that smoke particles or other airborne pollutants of small size when introduced into a detection chamber and subjected to a high intensity light beam will cause light scatter. The scattered light is sensed by a scattered light detector. The greater the amount of smoke particles within the sample introduced into the detector chamber the greater will be the amount of light scatter. The scatter detector detects the amount of scattered light and hence is able to provide an output signal indicative of the amount of smoke particles or other pollutant particles within the sample flow.
A difficulty arises in operation of smoke detector systems of the above kind in that most atmospheres where smoke or fire detection is required contain dust which may interfere with operation of the system. A filter may therefore be incorporated into the system for the purpose of keeping dust away from sensitive optical surfaces and to prevent dust from artificially affecting the detection of particles or gasses which are indicative of presence of fire of smoke. For example with optical scatter type detectors, the presence of dust may seriously affect the detector output since dust particles are generally larger than the particles which are indicative of the presence of smoke or fire and produce more scattered light than those particles.
A difficulty with the use of filters is that the filter will eventually fill with dust which can prevent passage therethrough of particles the presence of which it is desired to detect, so that the effective sensitivity of the detector system drops.